1. Field of the Invention
The present invention relates to a hot melt type ink jet head for melting solid ink and jetting the melted ink onto print paper for printing thereon. More particularly, the invention relates to an ink jet head capable of preventing the worsening of ink jetting performance attributable to the difference in thermal expansion coefficient of the components for the ink jet head.
2. Description of Related Art
One of the known types of ink jet head (abbreviated where appropriate to "the head" hereinafter) is the so-called thermal type head comprising ink chambers each having a nozzle and incorporating a heating element. In operation, suitably selected heating elements are energized and heated to produce air bubbles in the ink chambers. The pressure exerted by the generated bubbles causes ink to jet out of the nozzles.
Another known type of head is the so-called piezoelectric type head comprising ink chambers on top of which are furnished piezoelectric elements constituted by piezoelectric films made of piezoelectric material and by electrode films for applying voltages to the piezoelectric films. In operation, voltages are fed to suitably selected electrode films displacing the applicable piezoelectric elements through the piezoelectric effect generated therein. The displaced piezoelectric elements in turn change the volumes of the corresponding ink chambers, causing ink to jet out of the chambers through their nozzles. Piezoelectric head printers typically have a hot melt type ink jet head housing solid ink in an ink tank. The solid ink is melted by heat to be jetted out as liquid ink through the nozzles.
A typical hot melt type head that is driven piezoelectrically will now be described with reference to some of the accompanying drawings. Because the conventional head and the head of the invention are basically identical in structure, the drawings used to describe the inventive head are usefully referenced to describe the prior art hereunder.
FIGS. 2A and 2B are partially sectional views of the head according to the invention. FIG. 3 is a partially sectional view in effect when the head of FIGS. 2A and 2B is seen laterally.
A diaphragm (vibrating plate) 34 is made of an aramid film, such as of a highly aromatic polyamide fiber film. On top of the diaphragm 34 are piezoelectric elements 36 composed of piezoelectric material and secured to the diaphragm by an adhesive layer 37. Below the diaphragm 34 is a cavity plate 31 made of PES (polyether sulfone) and bonded by an adhesive layer 35 to the diaphragm.
A plurality of ink chambers 30 are formed within the cavity plate 31. Under the cavity plate 31 is a nozzle plate 32 made of nickel and fixed by an adhesive layer 48 to the cavity plate. The nozzle plate 32 has a plurality of nozzles 33 through which ink is jetted out. On top of the piezoelectric elements 36 is a base plate 50 which, made of alumina, supports the piezoelectric elements 36 and is secured by an adhesive layer 49 to the piezoelectric elements. Furthermore, as shown in FIG. 3, the piezoelectric elements 36 are topped with a heater 42 that keeps the ink melted in the ink chambers 30. When a suitably selected piezoelectric element 36 is energized, it is displaced through the piezoelectric effect generated therein so as to bend the diaphragm 34 into a downward convex shape, as described in FIG. 2B. The convexly deformed diaphragm changes the volume of the corresponding ink chamber 30 and thereby gives pressure to the ink therein. Under pressure, the ink is jetted out of the chamber through the nozzle 33 in the arrowed direction.
Meanwhile, the adhesive agent that forms the adhesive layers 35, 37, 48 and 49 typically has two properties. One is that the adhesive agent develops glass transition at a certain temperature (e.g., 124.degree. C.). The other one is that the adhesive agent deteriorates under the influence of heat.
Suppose that the head is heated by the heater 42 to a temperature (e.g., 125.degree. C.) exceeding the glass transition temperature (e.g., 124.degree. C.) of the adhesive. The trouble, in such a case, is that the adhesive agent softens through glass transition and loses some of its bond strength. This means that separations can occur between the components making up the head and lower the durability of the head. In particular, a lowered adhesive strength between the diaphragm 34 and the cavity plate 31 or the piezoelectric element 36 reduces the rigidity of these components. The reduced component rigidity in turn hampers the displacement of the piezoelectric elements 36 from being precisely transmitted to the diaphragm 34, thus deteriorating the ink jetting performance and lowering print quality.
As mentioned, the cavity plate 31, nozzle plate 32, diaphragm 34, piezoelectric element 36 and base plate 50 are all composed of different materials. As such, the components have different thermal expansion coefficients. In particular, the cavity plate 31 has a thermal expansion coefficient of 25.times.10.sup.-6, as opposed to 2.times.10.sup.-6 for the diaphragm 34. There is a considerable difference between the cavity plate 31 and the diaphragm 34 in terms of thermal expansion coefficient.
As the head is heated by the heater 42, the difference in thermal expansion coefficient causes the diaphragm 34 and cavity plate 31 to develop a significant thermal stress therebetween, greater than between any other components. As a result, the diaphragm 34 and cavity plate 31 are especially liable to separate from each other.
The problem caused from such separation, when taking place, is that: it can degrade the vibration characteristic of the diaphragm 34, lower the ink jetting performance, worsen print quality available with the head, and deteriorate the durability of the head. The disadvantages resulting from the difference in thermal expansion coefficient between the different components of the head are far more serious than those experienced with glass transition of the adhesive agent or with thermally induced performance deterioration. An urgent need has been recognized to circumvent the above-described problems.